402 



PHOTOMECHANICAL CONSIDERATIONS 



log K 



1.0 



0.5 



400 500 600 700 



800 ^ 900 



^ yuu m u 



Fig. 5. Reversible photobleaching of bacteriochlorophyll in methanol (air 

 not excluded). As shown in the insert, no reversibility is found after addition 

 of ferrous ions, while addition of ascorbic acid results in a reversibility with 

 a time response as observed with the slow component in chemical bleaching 

 (Fig. 2). A change in temperature affects both curves in the same way. 



Fig. 6. Difference spectra between bacteriochlorophyll and slightly photo- 

 bleached bacteriochlorophyll dissolved in methanol. before bleaching, 



-..-..- after bleaching, after addition of ascorbic acid and 15 minutes 



storage in the dark. 



versible. A difference spectrum between nonbleached and slightly 

 bleached bacteriochlorophyll is given in Fig, 6. 



According to Krasnovskii and Vonovskaja (7), who first discovered 

 this reaction, a peroxide test indicates that a peroxide is formed during 

 bleaching. 



